The genetic relationships among molecularly cloned prototype viruses representing all of the major oncovirus general were investigated by molecular hybridization and nucleotide sequence analysis. One of the major progenitors of the pol genes of such viruses gives rise to mammalian type C viruses and another gives rise to type A, B, D, and avian type C oncoviruses. The evolutionary studies were also extended to slow viruses such as caprine arthritis encephalitis virus (CAEV) and equine infectious anemia virus (EIAV). It was found that CAEV is homologous to intracisternal A particles and M432 viruses, while EIAV is homologous to human T-cell leukemia virus. Evidence of unusual patterns of homology among the env genes of mammalian type C and D oncoviruses illustrates that genetic interactions between their progenitors contributed to the evolution of oncoviruses. The human locus related to v-sis was cloned and shown to contain at least 5 exons corresponding to the v-sis coding region. Nucleotide sequence analysis of these exons revealed that the predicted amino acid sequence of human c-sis differed by 6% from that of the woolly monkey-derived v-sis. These findings imply that the sis proto-oncogene has been well conserved during primate evolution. By comparison of the known amino acid sequence of protein-derived growth factor peptides with the predicted human c-sis protein, it was possible to demonstrate that this human proto-oncogene is the structural gene encoding one of the two major polypeptides of this potent mitogen for connective tissue cells. The v-sis-related c-sis does not contain an initiation codon ATG. A putative exon upstream from the v-sis-related sequence was identified by Northern hybridization. Its sequence possesses an initiator codon ATG and splicing signal in phase with the downstream exon. This fragment, when ligated to the v-sis related sequences, enabled c-sis proto-oncogene to transform.